1. Field of the Invention
The present invention relates in general to the use of molecular beam epitaxy (MBE) formation of semiconductor layers. More particularly, the present invention relates to an improved method of monitoring the atomic flux from an effusion cell during MBE.
2. Description of the Related Art
MBE is a versatile technique for depositing single crystal semiconducting, insulating, and metallic materials used in fabricating state-of-the-art electronic and opto-electronic devices. The advantages of MBE over other growth techniques include the ability to produce high purity materials with controlled composition, layer thickness, dopant concentrations, and structure. MBE growth is achieved by directing the output of effusion cells onto a heated substrate in an ultra-high vacuum chamber. The effusion cell's output consists of atoms and molecules of the desired growth and doping constituents to be deposited.
Practitioners of MBE have long sought real-time control of the growth rates and composition of the deposited material. Although MBE offers the potential for growth of device structures with atomic layer precision, current "dead-reckoning" methods employed for controlling the effusion cell fluxes place limitations on the extent to which the desired composition, thickness and layer uniformity can be achieved. Real-time monitoring and control of these parameters hold the keys to achieving higher accuracy in attaining target growth structures and improved run-to-run reproducibility. Current generation MBE machines rely on pre-growth calibrations such as ion gauge flux measurements or RHEED oscillations to determine proper flux conditions. These methods are time-consuming, provide no real-time feedback, and are only marginally accurate when growing demanding structures. These problems add directly to device manufacturing costs.
A superior technique for in situ measurement of the flux of atomic species from an MBE effusion cell is known as optical flux monitoring (OFM). OFM detects changes in transmitted light intensity due to absorption by the atoms emitted from an MBE effusion cell. The technique has been employed in MBE to measure the flux of aluminum (Al), gallium (Ga), and indium (In) in real-time. Investigators at Sandia National Laboratories (SNL) have used OFM in a feedback-controlled system for growth of AlAs/GaAs quarterwave layers for distributed Bragg reflectors, as described in "Real-time Control of Molecular Beam Epitaxy by Optical-Based Flux Monitoring," by S. A. Chalmers and K. P. Killeen, in App. Phy. Lett. 63, 3131 (1993).
In the OFM technique developed at SNL, a feedback-stabilized hollow cathode lamp is used as a light source. The resonant radiation generated by the hollow cathode lamp is focused with a quartz lens onto the end of a sending fiber optic cable where it is divided into a reference beam and a signal beam. The signal beam is passed through the atomic beam to be measured. The signal beam enters the MBE chamber through heated optical ports (to prevent deposition of material on the window) and is directed parallel to, and a few centimeters above the surface of the substrate, to measure the flux of atoms reaching the substrate. After the signal beam exits the chamber, it is re-focused onto a second receiving fiber optic cable where it is measured by a detector. A separate detector measures the reference beam, and a computer calculates the flux level based on the ratio of the signal beam to the reference beam.
Another commercial OFM product, "ATOMICAS", manufactured by Intelligent Sensor Technology, Inc., was developed to eliminate problems due to transmission changes in the OFM optical system that are not due to flux changes of the atomic beam. This problem causes baseline instability that has plagued earlier attempts of MBE flux monitors that failed to use heated optical ports. In the "ATOMICAS" approach, optical radiation from a second source that is not absorbed by the atomic beam is passed through the signal and reference optical paths. The second optical radiation source consists of a xenon flash lamp with a broadband spectral output overlapping the spectral region of the emission from hollow cathode lamp. With this second radiation, the transmission changes of the OFM optical system are monitored in real-time. This information is used to calibrate the atomic flux measurements.
The above-described OFM techniques have several disadvantages. In both techniques, the atomic flux is measured near the substrate surface. This geometry is not optimal for several reasons. The foremost problem is that the transmitted light intensity is affected by atoms reflected or desorbed from the substrate surface. Monitoring of the atomic flux at close proximity to the substrate allows double counting of atoms if they have non-unity sticking coefficients. Also, the atomic flux is not monitored continuously. In the existing OFM approaches, there is no signal when the effusion cell shutter is closed. This necessitates "dead-reckoning" based upon effusion cell temperature data to infer initial flux conditions after the shutter is opened. An additional shortcoming of the existing OFM detection geometry is that, due to the divergent nature of the effusive source, the OFM signal is measured just above the substrate, where the number of atoms crossing the optical path is lowest. Finally, implementing OFM on existing MBE machines with the current geometry requires substantial re-working of the vacuum chamber and cryo-panels.